ISOLATION 723 



Many species of animal are probably so sedentary in habils 

 that the effective choice of breeding partner is small ; the excep- 

 tions will be wide-ranging active animals, such as many mammals, 

 birds and fish, and small species which have resting eggs or other 



Fig. 554. — Map of the British Isles showing the percentages of the bridled form 

 of the guillemot at breeding colonies in 1936-41. — From Southern and Reeve, 

 Proc. zool. Soc. Lond., Series A, 1941, in, 264. 



distributive stages which can be blown by the wind or carried 

 in other ways, for instance on the feet of birds. The melanic 

 moths which have evolved in the past century must have 

 been more or less sedentary ; if, however, the change in environ- 

 ment had occurred over the whole range of the species the trans- 

 formation could have taken place without isolation. In this way 

 one species may change into another, but cannot split into two. 



